The general principle of solar thermal technology is based on the concept of the concentration of solar radiation to produce generally steam, which is then used in standard power plants.
Solar energy collection, which has a relatively low density, is one of the greatest challenges in the development of solar thermal power plants. There are two types of solar concentrators: linear concentrators and point concentrators. Linear concentration is easier to install since it has fewer degrees of freedom, but it has a lower concentration factor and therefore it can reach lower temperatures than point concentration technology.
That is why an effort is made in order to progress in the development of the receiving tubes used in linear concentration, to try to increase the collection efficiency thereof and reduce thermal losses, such that the overall performance of the concentration plant increases.
The invention before us revolves around one of the elements that are part of said receiving tubes in particular, it is the expansion compensating device.
In general, a receiving tube consists of two concentric tubes between which the vacuum is produced. The inner tube, through which the fluid that is heated flows, is made of metal and the outer tube is made of glass, usually borosilicate.
The element described here, the expansion compensating device, is placed between both tubes so that it allows the movement of the tubes lengthwise and ensures the vacuum, absorbing the stresses created by the difference between the coefficients of thermal expansion of metal and glass.
In the state of the art several developments for this element are known, but the one that offers the best results is disclosed by SCHOTT in the patent U.S. Pat. No. 7,013,887. In said document the expansion compensating element consists of a folding bellows that binds to the metal tube by a connection element and to the glass tube by a glass-metal transition element.
Said connection device has a number of drawbacks. One of them is the decrease in the performance of the system because a longer or shorter glass tube portion is covered inside by this bellows, such that no solar radiation penetrates the metal tube.
Another drawback is derived from the height of the waves of the bellows. The height of these waves is what determines the diameter that the borosilicate tube should have, because the expansion compensating device is located inside of it. Therefore, the greater they are, a greater tube diameter is required, which makes the product more expensive and increases heat losses.
Therefore, the present invention is intended to devise a new compensating device that meets the requirements for this type of elements and at the same time improves its performance with a view to the existing products on the market.